MATHS PUZZLE # 9 FIRST LEG (Second Year)

(from the 29th of November to the 6th of December)


 JULIA'S FIRST DAY AT SCHOOL
It’s the first day of school for Julia. The teacher, Miss Rivas, has told her students to meet in the playground. She says: “You must all shake hands and introduce yourselves”

If there were 10 students in her class, how many handshakes were there?

You can send your answers to matecampanar@gmail.com

SOLUTION

The first student will shake hands with 9 students, the second student with 8, the third student with 7 and so on. So we have 9+8+7+6+5+4+3+2+1 =45

Or another way to solve the problem:

If there are 10 students, each one will give his/her hand to the other 9 students. That is 10 x 9 =90. However, if we calculate the handshakes this way, we are counting each handshake twice: for example when say B greets C and when C greets B. Thus the number of handshakes is half the calculations made above, therefore (10 x 9)/2=45